(a) Field of the Invention
The present invention relates to an attachment lens system which is attached on the image side of a photogramic camera lens system for prolonging its focal length.
(b) Description of the Prior Art
An attachment lens system which is used for prolonging focal length of a master lens system has a negative refractive power as a whole and is placed in most cases close to the imaging position of the master lens system in order to avoid mechanical interference with the master lens system. Therefore, the attachment lens system has a strong negative refractive power and a negative Petzval's sum large in absolute in the entire lens system of the attachment lens system, thereby shifting imaging position largely toward the positive side in the entire lens system including the master lens system as angle of field is increased. Further, the attachment lens system has an effect to aggravate aberrations in the master lens system. Especially when spherical aberration has a large negative value, the optimum imaging position at each field angle is largely shifted along optical axis. Furthermore, since the attachment lens system has a large negative refractive power and aberrations in the attachment lens system itself can hardly be corrected, image quality is apt to be rather degraded as compared with that in the master lens system when the attachment lens is combined with the master lens system. There have conventionally been known attachment lens systems disclosed by Japanese unexamined published patents No. 109930/77 and No. 146621/77 and having compositions similar to that of the attachment lens system according to the present invention described later in this patent specification. The former attachment lens system comprises five components of seven lens elements, but cannot assure favorably corrected astigmatism regardless of the larger number of lens elements and seems to have undercorrected Petzval's sum. The latter attachment lens system comprises four components of six lens elements, seems to be designed for use with a master lens system having a focal length of 300 mm and F2.8, and has aberration more aggravated when it is combined with a tele photo lens system having focal length of 100-200 mm. As is described in the foregoing, the conventional attachment lens systems had defects that Petzval's sum and other aberrations were not corrected sufficiently and quantity of marginal ray is apt to be insufficient.